Preliminary investigation with four pairs of young rats has shown that when an intact male is parabiosed to an oophorectomized female and a kidney is removed from each, the serum LH rises sharply in the female, testosterone rises in the male partner. In a similarly-treated group of four pairs allowed to live 16-20 ms., all four male partners developed unilateral interstitial cell tumors of the testes, and three of four developed small adenocarcinomas of the prostate. We propose to confirm the above with a larger series to study the endocrine changes in more detail, and to test the effectiveness of Danazol and Cyproterone in preventing these tumors.